1. Field of the Invention
The present invention relates to a matrix liquid crystal display.
2. Description of the Related Art
In the case where the liquid crystal is driven by applying to it a video signal in conventional matrix liquid crystal display, an AC drive is required in which positive and negative voltages are alternately applied to a common electrode of the liquid crystal in order to prevent the deterioration of the liquid crystal. FIG. 1 is a block diagram showing a liquid crystal drive circuit used for a conventional matrix liquid crystal display configured as an integrated circuit.
The liquid crystal drive circuit shown in FIG. 1 includes a shift register 21, a first latch circuit 22 for latching n-bit video data from the shift register 21 in parallel, a second latch circuit 23 for latching the data output from the first latch circuit by a latch signal, a decoder 24 and a level shifter 25 for selecting 2.sup.n gray-level voltages supplied the n-bit video data from an external source, and 2.sup.n analog switches 26 (JP-A-63-304229).
Each output terminal of the liquid crystal drive circuit selects one value from the 2.sup.n gray-level voltages and applies a predetermined gray-level voltage to the liquid crystal. In the process, in order to drive the liquid crystal in AC drive, the gray-level voltage applied to the liquid crystal has conventionally been changed for each line or each frame of the matrix liquid crystal.
In this liquid crystal drive circuit, a voltage twice as high as the threshold voltage of the liquid crystal is required for applying positive and negative voltages alternately to a common electrode of the liquid crystal. The threshold voltage of the liquid crystal is normally about 4 to 5 volts. For AC drive, therefore, the liquid crystal drive circuit is required to have a breakdown voltage of at least 10 V. In view of this, a diffusion process of high breakdown voltage has conventionally been used for fabricating an integrated liquid crystal drive circuit.
In the case where the liquid crystal drive circuit for the matrix liquid crystal display shown in FIG. 1 is fabricated as an integrated circuit, the use of a diffusion process of high breakdown voltage poses the problem of a large chip size. This is attributable to the fact that the diffusion process of high breakdown voltage requires a long gate, a thick gate oxide film and a low-concentration layer for increasing the breakdown voltage of a transistor. Further, component elements are required to be isolated from each other, thereby leading to a large transistor size.
Further, when the liquid crystal drive circuit shown in FIG. 1 is fabricated as an integrated circuit, a long diffusion process leads to the problem of a higher chip cost. The reason is that the recent trend toward a matrix liquid display apparatus of higher definition is so marked that the logic unit of the liquid crystal drive circuit requires a high operating speed of at least 40 MHz. Also, the driver unit, which AC drives the liquid crystal, requires a breakdown voltage of not less than 10 V. As a result, a process of a low breakdown voltage (5 V) mixes with a process at a high breakdown voltage (10 V or more), so that the diffusion process is longer than the process at low breakdown voltage.
Another problem of the prior art is a large power consumption. This is because a voltage at least twice as high as the threshold voltage of the liquid crystal is required to be applied to the voltage source of the liquid crystal drive circuit.